1. Field of the Invention
The present invention pertains to a manufacturing method for couplers for optical fibers employed in communication systems, data transmission equipment, optical sensors and the like, which are used for joining light signals and at optical fiber branch points.
2. Prior Art
In the past polarization maintaining optical fiber couplers have been manufactured as will be described below. First of all, two component polarization maintaining optical fibers 12A, 12B are mutually aligned side by side so that their longitudinal axes are parallel with respect to one another, and additionally, so that their X-axes 15 are parallel with respect to one another, as is shown in FIG. 1. As can be seen in FIG. 1, each component optical fiber 12A, 12B includes a core 16 and cladding 18 in addition to the above mentioned stress applying parts 14.
Next, as is shown in FIG. 2, a portion of the aligned component optical fibers 12A, 12B is thermally fused, and the fused portion 20 thereby formed is then drawn out to form a fused-elongated region 11 as is shown in FIG. 3, whereby the optical fiber coupler 10 is manufactured.
Conventionally, with such a manufacturing method as described above, in order to mutually align the stress applying parts 14 of each component optical fiber 12A, 12B, a measuring system as is shown in FIG. 4 has been employed. In this measuring system, the component polarization maintaining optical fibers 12A, 12B are immersed in a matching oil having an index of refraction identical to that of the cladding 18 of the component optical fibers 12A, 12B, whereby refraction at the surface of the claddings 18 is eliminated. A light source 24 is placed below the component optical fibers 12A, 12B, and they are then examined from above using a microscope 26.
Since the index of refraction of the stress applying parts 14 differs from that of the claddings 18, the transmitted light from the light source 24 is refracted at the interface of the stress applying parts 14 and the claddings 18, the forms of the stress applying parts can be seen or be observed by the microscope 26. Thus, while under observation, the component optical fibers 12A, 12B are rotated about their longitudinal axes, whereby the stress applying parts 14 are brought into alignment.
With this type of alignment method however, the accuracy of the obtained alignment is very much dependant on the skill of the individual. For those not skilled, there tends to be great variability in the quality of the obtained optical fiber coupler. In the case where angular deviation between the stress applying parts is present in the manufactured optical fiber coupler, cross-talk characteristics suddenly degrade. The relationship between angular deviation of the stress applying parts and cross-talk is shown in FIG. 5. It can be seen from the drawing that in order to limit cross-talk in this type of optical fiber coupler, it is necessary to limit angular deviation as much as possible.